Traditionally, spectacle lenses were made from glass or, more recently, cast thermoset plastics such as CR-39 (allyl diglycol carbonate, cast-polymerized with a peroxide cure). Just in the last two decades, however, the spectacle lens inductry has found they can get better lenses cheaper by highly-automatable injection molding of thermoplastics (most specifically, optical-grade polycarbonate) with highly-automatable liquid dipcoating and curing. For example, twenty years ago the world market for non-vision-corrective ("zero-power" or "plano" lenses) spectacles for industrial workers' safety was still dominated by heat-tempered glass lenses. Now, this market is dominated by hardcoated polycarbonate plano lenses, which can be made for a fraction of the cost of the glass lenses and, at the same time, offer better worker eye protection, by providing far greater breakage resistance (polycarbonate has 10-20 times stronger than glass' impact strength) and better UV-absorbancy protection. The result is that in the U.S., about 85% of all zero-power, industrial safety spectacle lenses are now hardcoated polycarbonate. These lenses' hardcoatings are very much a mature commodity, so lower cost and improved ease of use is important, while maintaining film hardness. General chemistry of these nontintable heat-curing silicone hardcoats are copolymers of monomethyl silane with colloidal silicas. (Applicant Galic is a co-inventor in a co-pending patent application U.S. Ser. No. 08/430,251 now U.S. Pat. No. 5,665,814, on improved versions of same.)
For vision-corrective ophthalmic prescription spectacle lenses, there are other needs and other considerations which have to be met. These hardcoating deficiencies have historically retarded market share growth of polycarbonate Rx spectacle lenses. One such need is for tint-receptivity--the ability for optical laboratories or dispensers to immersion-dye the hardcoated lens in aqueous dye color solutions, to achieve either a lightly-colored fashionable cosmetic tint or, alternatively, a darkly-colored sunglass tint. In the past, lack of tintability of the hardcoatings have held back polycarbonate Rx lenses from widespread acceptance, particularly in the U.S. market, where the majority of Rx lenses are tinted in some way. These tinting operations are conducted in hot (typically, 195.degree.-205.degree. F.) aqueous dye baths, so the hardcoating must be able to accept coloring while, at the same time, maintaining water-resistant adhesion onto the polycarbonate lens substrate and resisting thermal shock/embrittlement of the coating, such as cracking or crazing, as the lens (at room temperature) is dipped into the hot dye bath, and subsequent rinsing in cool water after removing the hot lens out of the dye bath.
Another requirement for Rx spectacle lens hardcoatings not properly met by the monomethyl silicone or methyl silane/silica copolymer nontintable hardcoats is compatibility with anti-reflective vacuum coatings. Such monomethyl silane/siloxane hardcoats have a refractive index of about 1.43; polycarbonate has a refractive index of 1.586. These anti-reflective coatings are applied by various vacuum deposition processes (such as thermal evaporation, or sputtering, or ion-assisted depositions), and these AR coatings on any plastic lens substrate have much better mechanical durability if a good liquid hardcoating of at least 2-6 microns' thickness is first applied as a base coat before AR coating is deposited. In Japan and other Far East oriental markets, the vast majority of plastic Rx lenses are hardcoated, then AR topcoated in this way. Once AR coated, the lens cannot be tinted, but in these markets, a much smaller percentage of Rx lenses are colored at all (typically, less than 20%). Similarly, in Europe, a majority of Rx glass and high index plastic lenses are AR coated, and relatively fewer are tinted colored. Due to the refractive index mismatch of the monomethyl-silane-based coatings with polycarbonate, relatively poorer anti-reflective coating performance results from such an AR-coated/monomethyl silicone-coated polycarbonate lens. Therefore, a hardcoating having a refractive index much closer to that of polycarbonate is desirable as a base coat for AR-coated polycarbonate Rx lenses. AR coating also involves thermally cycling processes, so brittleness is a worry, and although the AR coating film is very thin (far less than 1 micron), it is also very brittle, thus further stressing the hardcoat and substrate boundary layer adhesions and water resistance bonds. Thus, the thermal shock of the water boil test (lens surface temp from room temp to 100.degree. C. nearly instantaneously) and/or cyclic humidity tests are found to be predictive of field failures due to cracking or delamination of these coatings.